Rec'd letter from IRS to pay penalty for "not pre-paying tax" (??); initially i set to pay by 4/15 but paid on 3/8; IRS charged me automatically on 4/15. what do i do now

Ok ... if you did not pay enough in estimated taxes during the year the IRS can and will assess an underpayment  penalty after you have filed.   TurboTax only computes an estimated penalty. 

Then you set up the payment to happen by debit on 4/15  but paid it on 3/8 instead ... the mistake was that you did not call the IRS and stop the payment.  The excess payment should be refunded automatically once the IRS figures out what you did ... 

Notice of Overpayment Letter

In some cases, the IRS may identify the overpayment. When this happens, the IRS sends the taxpayer a CP 268 notice in the mail. If you receive one of these notices, read it carefully. It will explain the situation and advise you as to what actions you need to take.

Typically, you'll be asked to call the IRS directly and speak to one of its agents. The agent will give you your options, which may include requesting a refund check or simply applying the amount to next year's tax debt.

Request Overpayment of Taxes Refund

If you receive notification of your tax overpayment from the IRS via a CP 268 notice, you'll be able to request your refund directly from an IRS agent over the phone. If you identify the overpayment on your own, you should file an amended tax return for the year or years in which you overpaid. If the IRS agrees with your assessment, you'll receive your refund.

Refund Time Limit

The IRS has set limitation periods on claiming tax refunds. The last claim date for tax return is the latter of three years from the time you filed a return or two years from the time you actually paid the tax.